Scaling Tendencies

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What is Scaling?

Scaling is the deposition of a mineral salt on processing equipment. Scaling is a result of supersaturation of mineral ions in the process fluid. Through changes in temperature, or solvent evaporation or degasification, the concentration of salts may exceed the saturation, leading to a precipitation of solids (usually crystals).

Solubility Product Constant, K_sp The solubility of ionic compounds of salts and minerals in water are governed by a solubility equilibrium expression and a solubility product constant known as K_sp. It is important to note that the solubility product, K_sp is a function of both temperature and pressure. Consider the general dissolution reaction below (in aqueous solutions):


With equilibrium constant K_sp defined as: K_sp=〖(a〗_B )^b ∙(a_D )^d

Where, a_B and a_D are the activities of the aqueous species. The activity of any species i is defined as the product of its concentration in molality by its corresponding activity coefficient:

a_i=m_i γ_i

Ion Activity Product, IAP A real solution may not be in the state of equilibrium. This non-equilibrium state is described by the ion activity product (IAP). It has the same form as the equilibrium constant K_sp, but involves the actual activities of the species in solution. IAP=(a_B )_actual^b∙ (a_D )_actual^d

Scaling Tendency and Pre-scaling Tendency in OLI Software

Scaling Tendency is the saturation ratio after all potential solids come to equilibrium with water. This is the true equilibrium condition (time=∞). In OLI Software the Scaling Tendencies are reported under the name of Post-Scaling.

Pre-scaling Tendency represents the condition before any solids are allowed to form. This is a non-equilibrium condition and can be viewed as the condition where time=0.

Scaling Tendency (Reported in the software as Post-Scale)

Scaling tendencies are essentially saturation ratios. When the scaling tendency for a species is 1.0, it indicates that its solid form is in equilibrium with water. A scaling tendency below 1.0 indicates sub-saturation and that the solid phase will not form.

Scale Index (Reported in the software as SI, Index)

Scale Index (SI) is the log value of the Scaling Tendency (ST):

SI = Log (ST)

There are three scenarios for the Scale Index (also referred to as the Saturation Index)

  • SI = 0 IAP = Ksp → saturated (in equilibrium)
  • SI < 0 IAP < Ksp → undersaturated
  • SI > 0 IAP > Ksp → supersaturated

Pre-Scaling Tendency (Reported in the software as Pre-Scale)

OLI software defines a Pre-Scaling tendency as the scaling tendency before solids precipitate. The Pre-scaling tendency is calculated the same as the Post-scaling tendency:

ST = IAP/Ksp

Pre-Scaling Index (Reported in the software as SI, Index)

Pre-Scale Index (SI) is the log value of the Pre-Scaling Tendency (PST):

PSTI = Log (PST)

Note: Many industries, notably the up-stream oil & gas industry, use the pre-scaling tendency to make design decisions about adding anti-scaling and anti-fouling agents or if the asset is as risk.

Difference between Post-scale and Pre-scale

  • Pre-Scale: The saturation ratio before solids precipitate.
  • Post-Scale: The saturation ratio AFTER solids precipitate (if solids are selected).
  • S, ST – Saturation, Scale Tendency: The ratio of the concentration (activity) to its solubility (S=1).
  • SI – Scale Index: Log(S).

Calculating Scaling Tendency: An Example

Below there is an example of how the software calculates the Scaling Tendency. Consider the dissolution of sodium bicarbonate:

NaHCO3(s) = Na+ + HCO3-

Scaling Tendency is defined as ST = IAP/Ksp

Where IAP is the Ion Activity Product, and is defined as the product of specific ions (in this case the ions resulting from the dissociation of a particular solid); and Ksp is the Solubility Product Constant which gives the thermodynamic limit of ion availability.

Let’s consider a 1.0 molal NaHCO3 solution. The IAP is calculated as follows:

IAP = [gammaNa+]*[mNa+]*[gammaHCO3-]*[mHCO3-]

Assuming Ideal Solution Activities:

gammaNa+ = 1.0

gammaHCO3- = 1.0

mNa+ = 1.0

mHCO3- = 1.0


IAP = (1.0)(1.0)(1.0)(1.0)

IAP = 1.0

Defining the Ksp

Ksp = [gammaNa+]*[mNa+]*[gammaHCO3-]*[mHCO3-]

Ksp = 0.403780

The Scaling Tendency is then the ratio of available ions to the thermodynamic limit:

ST = IAP/Ksp

ST = 1.0/0.403780

ST = 2.48

Was assuming ideal conditions valid?? The actual species concentration and activity coefficients (calculated by the OLI software) are:

gammaNa+ = 0.598

gammaHCO3- = 0.596

mNa+ = 0.894

mHCO3- = 0.866

This results in a different IAP:

IAP= (0.598)(0.894)(0.596)(0.866)


The new Scaling Tendency is the following:

ST = IAP/Ksp

ST = 0.276/0.40378

ST = 0.683